Shadow masks of picture tubes are usually surrounded by a frame. This combination mask/frame is set in the shielding case of the picture tube. The joint between shielding case and combination mask/frame is implemented by means of "pins", set in the shielding case, the free ends of which contact the free ends of springs attached to the outer contour of the frame in such a way that a portion of the length of the pins projects through an opening arranged in each spring. The other ends of the springs, which are bent at an angle but run essentially parallel to the contour of the frame, are welded to the frame. Usually a bimetallic retainer is interposed between the welded end and the frame in order to compensate for thermal expansion during operation of the picture tube.
To obtain good reproducibility in the position of the combination mask/frame when the combination mask/frame is snapped in and out, essentially two procedures have emerged in the prior art:
In the first procedure, three of the four springs are welded onto the frame and onto the bimetallic retainers, and the combination mask/frame is set in the shielding case by means of the three springs. Then the opening of the fourth spring is set on the free end of the pin, and the end to be attached to the frame is snapped into place behind a hook located on the bimetallic retainer. The spring that snaps into place behind the hook is then joined to the bimetallic retainer by resistance welding.
According to the other procedure, the three springs already described above are again welded to the frame and to the bimetallic retainers. In addition, during this procedure the fourth spring is also resistance-welded to the bimetallic retainer. This fourth spring has an opening that is larger than the openings in the other three springs. When a combination mask/frame configured in this manner is then set in the shielding case, the combination mask/frame is held in the shielding case by the three springs having the smaller openings, which rest on the pins. After the combination mask/frame is snapped into place, the pin corresponding to the fourth spring, which has the larger opening, also projects through it. However, because of its larger opening cross section as compared to the openings of the other springs, this spring does not sit on the outer contour of the pin, but surrounds the pin at a distance. The free end of the spring is then bent towards the frame and a centering washer is slid onto the pin. This centering washer has an opening which corresponds, in terms of shape and dimension, to the openings of the three openings sitting on the outer contours of the pins. When the spring is then relaxed, the free end of the spring presses the centering washer against the pin. The result is that the fourth spring sits on the outer contour of the pin via the centering washer, and a portion of the length of the pin projects through the opening of the centering washer and the spring. In this state the spring is resistance-welded to the centering washer.
The latter method has the disadvantage that resistance welding causes spatters which burn into the glass of the shielding case and thus render the case unusable. The welding spatter problem also occurs with the first method. Also considered to be a disadvantage with the first method is the fact that in order to join the fourth spring to the frame and the bimetallic retainer, welding tongs must be applied to the frame for resistance welding. The frame can very easily be bent by the welding tongs, and can also be bent by thermal stresses in the frame and the spring as a result of the welding process. If combination mask/frames produced by the aforesaid procedures are subjected to closer examination, it is evident that the expansion characteristics of the combination mask/frames during operation of the picture tube are not the same at the fourth spring as at the other three springs. This may still be tolerable with smaller picture tube formats, but is no longer tolerable at larger formats with a mask/frame contour that has little curvature.
The underlying object of the invention is therefore to indicate a process for designing combination mask/frames that allows the production of combination mask/frames with uniform expansion characteristics.